The present invention relates to paper-making machines and in particular to twin-wire machines wherein drainage or dewatering of the paper web during formation of the latter takes place to a large extent simultaneously through two wires one of which is a web-carrying wire and the other of which is a web-pressing wire, the web-carrying wire being that one of the pair of wires which continues to carry the web after the pressing wire detaches itself from the web. The web-carrying wire forms a loop within which are situated pair of suction rolls. The first of these rolls in the direction of travel of the web-carrying wire is a forming roll while the second of these rolls is a couch roll. The web-forming roll has a section sector for enhancing dewatering, and this latter sector extends through an angle of at least 90.degree. and includes at least one suction compartment covered by both wires, with dewatering primarily achieved by the pressure between the wires.
Suitable guide rolls guide the web-carrying and web-pressing wires respectively along converging paths which meet at the forming roll at the beginning of the common path of travel of the pair of wires, and these converging paths of the pair of wires define a throat into which the stock in the form a of a suitable fiber suspension jet is delivered by the headbox.
From the beginning of the common path of the wires at the suction sector of the forming roll, the wires travel along this common path upwardly and preferably obliquely to the upper couch roll with the common path continuing at least up to the point where the carrying wire first contacts the couch roll.
In recent years there has been the development of a number of twin-wire machines wherein drainage from the web during formation thereof is attempted in a number of different ways so as to bring about dewatering simultaneously in opposed directions. It is clear that the speed with which dewatering takes place is appreciably increased by expedients of this type. Furthermore, the paper web which is produced in this way has more homogeneous structure than a paper web produced on a conventional Fourdrinier wire.
When twin-wire machines were first used, the objective was to achieve a symmetrical dewatering and the common path of the wires was vertical with the stock being fed to the upper end of the common path. However, twin-wire machines used at the present time differ from these earlier constructions by such features as the manner in which the stock suspension is delivered to the wires and the situation of the common path of travel of the wires, the web formation of course taking place while the web is compacted between the wires as they travel along their common path.
It has been found to be advantageous in most cases to increase the extent of dewatering by utilizing centrifugal force. This result is achieved by designing the twin-wire machine in such a way that the wires are compelled to travel along a curved path, particularly at an initial portion of the common path of travel of the wires, the latter initial portion being situated immediately subsequent to the headbox, and in this initial portion where centrifugal force is utilized at least in part the greatest part of the dewatering takes place.
A construction of the above general type is shown in British patent 993,837 wherein the wires are disclosed as passing around two rolls so that the common path of the wires is of a wave-shaped or S-shaped configuration.
However, this type of construction and variants thereof have the drawback that that one of the wires which directly engages the first roll is an inner wire at the first roll while at the second roll this same wire forms an outer wire which is not directly in engagement with the second roll. The result is that the wires necessarily undergo a displacement with respect to each other with an unavoidable damage to the fiber web which is in the process of being formed between the wires. A construction of this type disturbs the orientation of the fibers in the web and reduces the strength of the web.
There are also known constructions wherein both wires at their common path curve in one direction only, and of course the web which is formed between the wires curves in the same direction. U.S. Pat. No. 3,438,854 is exemplary of such constructions. However, such constructions have the drawback that the curved path of the wires is created by utilizing a relatively large number of differently constructed components which frictionally rub against the wires, these components, for example, having the form of a curved suction shoe or the like. Such components have a pronounced abrasive effect on the wires, and in addition they are disadvantageous in that considerable space is required for such components in the longitudinal direction of the paper machine.